Paraffins are hydrocarbons solid at room temperature, consisting of saturated straight, branched and/or cyclic hydrocarbon chains. Paraffins based on mineral oil contain about 20 to 50 carbon atoms whereas other, particularly synthetic, paraffins contain up to 100 carbon atoms. Owing to their origin, e.g. from different deposits of mineral oil, the number of carbon atoms and the extent of branching vary widely. Paraffins are roughly classified, depending on their solidification temperature, as low-melting soft paraffins having a solidification point (Ep) between approximately 30 and 48.degree. C. and higher-melting hard paraffins having a solidification point between approximately 50 and 65.degree. C. Paraffins also include microcrystalline paraffins (microwaxes) from distillation residues or heavy distillates of paraffin-based crude oils.
The value of grades of paraffin is proportional to their melting point. Depending on their purity and solidification point, paraffins have a variety of uses, particularly in the candle, paper, rubber, packaging, textile and food industries. They are also used for producing maintenance and polishing agents, cosmetic and pharmaceutical products and latent heat storage material.
The production of paraffins starts from crude paraffin, which occurs as a by-product of lubricating-oil production in crude oil refineries, and has variable proportions of liquid components, and solid paraffin fractions obtained by thermal degradation of plastics are another source. The oil content is the name given to all components of paraffin or crude paraffin which are liquid at room temperature. The final paraffin, depending on the required quality, must usually have a maximum oil content of 0.5 to 1.5% by weight. In the case of the finished product containing not more than 0.5% by weight of oil, the solidification point and the needle penetration value are additional quality features.
On a large industrial scale, there are two different basic kinds of known methods of de-oiling high-quality paraffin from paraffin-containing fractions occurring in refineries. Sweating de-oiling is the earlier method whereas solvent de-oiling is the later and more efficient method.
In 1959 F. Asinger, "Chemie und Technologie der Paraffin-kohlenwasserstoffe", 1st edition, Berlin, page 46, made the following comment on sweating de-oiling:
"The sweating process is difficult to control in practice and in future will probably be replaced by solvent de-oiling."
Sweating de-oiling has in fact become obsolete owing to its serious disadvantages, and solvent processes are mainly used today. According to "Ullmanns Enzyklopadie der technischen Chemie", Volume 24, page 26, in the USA over 90% of existing plants operate on the principle of solvent de-oiling. No new sweating de-oiling plants have been built for some time.
Solvent de-oiling processes, also called solvent de-oiling or selective de-oiling, are continuous processes whereby paraffins can be obtained from all paraffin-containing melts and broken down into fractions.
Solvents are used for separating paraffins and oil components and the yield of paraffin is better than in sweating de-oiling. Solvent de-oiling processes are characterized by high flexibility when using different grades of crude paraffin, since the solvent composition and the ratio of crude paraffin to solvent can be altered so as to deal with different kinds of crude paraffin. The following solvent de-oiling processes are used:
1. The mash method, wherein the mixture of crude paraffin and solvent, directly after paraffin removal, is mashed with additional solvent and then filtered. PA1 2. The precipitation process or crystallization process wherein the liquid crude paraffin is dissolved in the solvent, after which the solid paraffin is precipitated by cooling, and PA1 3. The spray process, wherein the melted crude paraffin is sprayed as a powder in a counter-current of cold air and then mixed with solvent. PA1 no endangering of the environment or health, PA1 no corrosion of plant, and PA1 much lower operating and maintenance costs. PA1 increases the efficiency of the plant and PA1 significantly improves selectivity, which PA1 increases the yield of high-quality paraffin. PA1 crude paraffin having a relatively high oil content can be processed PA1 as a result, run-off oil can be more profitably processed by partial recovery of the paraffin components, and consequently PA1 the total yield is significantly improved.
The difficulties with all solvent processes are due to the environmental and health-endangering properties of the solvents, which are used in large quantities (3 to 10 parts of solvent per part of crude paraffin) and their corrosiveness and high flammability. Also considerable energy is needed to recover the solvents and for facilitating the process by cooling. In spite of these problems and cost factors, solvent de-oiling has now ousted sweating de-oiling because it yields more paraffin and increases the capacity of the plant and because paraffin-containing melts with a higher content of micro-crystalline paraffins can also be de-oiled.
Sweating de-oiling was hitherto the only known process of use on a large industrial scale for substantially solvent-free de-oiling of paraffin. This property of sweating de-oiling is very welcome for ecological reasons and has a positive effect on the plant, maintenance and operating costs. The process is carried out in chambers equipped with horizontal heat exchanger spirals or vertical banks of heat exchanger tubes and horizontal perforated trays. A preset amount of water is first poured into the chambers or containers so as not to clog up the outlets, after which the melted crude paraffin is poured into the chambers. The crude paraffin floats on the water. The crude paraffin is then cooled to form a solid block, after which the water can be let out under the solid block, which rests on heat-exchanger lines and/or screen-like intermediate trays. The block is then slowly heated, when the oil, and also low-melting soft paraffins at higher temperature, are sweated out of the paraffin block. The outflowing liquid phase is called run-off oil. Owing to the equilibrium in solution, it can contain high proportions of the higher-melting paraffin components, which are the substance to be obtained.
In "Ullmanns Enzyklopadie der technischen Chemie", Volume 24, page 26, the disadvantages of sweating de-oiling are described as follows:
"No new plants are being constructed for this conventional de-oiling process, owing to the low selectivity (poorer yields of hard paraffin), the time-consuming heating, the discontinuous operation and the non-applicability to very oil-binding crude paraffin from heavy machine-oil distillates. Attempts are made to improve the yield of hard paraffin from existing plants by partial recycling of the run-off oil".